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Shell And Tube

The aluminium refining process places many unique challenges on heat exchanger design and fabrication . This paper addresses some of the those features that handle the heating and cooling of super saturated solution downstream of the digester . Precision Equipments demonstrates how such requirements can be addressed .

By Ganesh Prasad ( Technical Sales ) and Prabhu Bala ( Company Director ) - Precision Equipments

Unlike in the conventional processes , heat exchangers used in aluminium refining are subjected to a very different process environment . One such application is the digestion operation in a typical Bayer process .

Aluminium growth Aluminium is a silvery-white lustre element which is non-magnetic and ductile in nature . It is the most abundant metal in the earth ' s crust . This metal has high demand in the field of aerospace , packaging , and electrical instruments . Increasing demand for electric vehicles and OEM manufacturers is supporting the growth of the aluminium market . The demand for packaging foods , coupled with the rising replacement of stainless steel with aluminium by automotive industry is acting as a market driver . Nevertheless , disruptions in the supply chain due to short-term shutdowns of mines are reflected to have negative effects on the market growth . Aluminum in its purest form is soft and weak . Commercial aluminum with small traces of silicon and iron will be hard and strong . Aluminum plays a vital role in the construction of aircraft and aerospace applications , marine applications , food packaging , machinery equipment and electrical applications because of its high resistance to corrosion and lower density .

Aluminium is extracted from bauxite ore , accomplished by 2 phases . Aluminium oxide is extracted from the ore using Bayer process and aluminium is extracted from that aluminium oxide using Hall- Heroult process .

The Bayer Process Bauxite ore is mechanically crushed . This crushed ore is mixed with caustic soda and processed in a grinding mill to produce slurry containing very fine particles of ore . The slurry ( sodium aluminate solution ) is pumped into a digester tank and heated to 100-200 ° C under a pressure of 340 kPa using critical shell & tube heat exchangers with multi pass radial tube layout , larger tube sizes and an expansion bellow on the shell side . These conditions are maintained for several hours and then the hot slurry passes through a series of flash tanks that reduce the pressure and recover heat that can be reused in the refining process . The slurry is pumped into a settling tank . So , the undissolved impurities are settled to the bottom of the vessel and the aluminium remains dissolved in the caustic soda . The residue that accumulates in the bottom of the tank consists of fine sand , iron oxide and oxides of trace elements like titanium . After the impurities have settled out , the remaining liquid is pumped through a series of cloth filters . Any fine particles or impurities that remain in the solution are blocked by the filters . This material is washed to recover aluminium oxide and caustic soda that can be reused . The filtered liquid is pumped through a series of crystallization tanks . Seed crystals of aluminium oxide hydrate are added through the top of each tank . The seed crystals grow as they settle through the liquid and dissolved aluminium oxide attaches to them . The

Crystallization

H ₂

O

Aluminium oxide

Rotary Kiln

Figure 1 : Flow chart of the Bayer process

50 Heat Exchanger World September 2022 www . heat-exchanger-world . com